Nutritional and environmental impacts on skin and mucus condition in Atlantic salmon (Salmo salar L.)

Abstract

The skin and associated mucus layer of Atlantic salmon constitutes its first line of defence against the aqueous environment. Through intensive farming, a range of stressors including both mechanical and environmental factors are known to have an impact on the skin condition of fish. Damaged skin can serve as a portal of entry for primary pathogens and secondary infections. Two of the current main problems in the salmon farming industry are skin related: ectoparasitism with sea lice and skin wounds of diverse origin, often related to extreme temperatures. Four experiments with salmon were conducted to investigate the response of skin and the epidermal mucus layer to challenges representative of normal farming conditions, and to assess the possibility to modulate the skin response through diet. The factors studied included water temperature change, sea lice infection and mechanical wound infliction. New methodologies were developed or existing methods optimised to assess the impact of these factors on the composition, structure and functionality of skin. Quantitative histology using digital image analysing, proteomics of mucus and transcriptomics of skin were all demonstrated to be valuable tools in defining differences between groups exposed to distinct treatments. The chemical composition of the skin was studied at 3 temperatures; 4°C, 10°C and 16°C. Increasing temperature resulted in higher level of protein and collagen related amino acids, while moisture level was reduced. Altering the dietary levels of minerals and vitamins also resulted in changes in their concentration in the skin, demonstrating dietary modulation of skin composition. The structure of the epidermis was also affected by temperature. Quantitative histology assessments showed that the epidermal thickness decreased from low to high temperature, whereas the epidermal area comprising mucous cells increased. Temperature also impacted on the skin transcriptome. A subtle increase in skin-mediated immunity was observed at low temperature, suggesting a pre-activation of the skin mucosal immunity. Up-regulation of a number of heat shock proteins correlating with a decrease in epidermal thickness, was consistent with a stress response in the skin of fish exposed to high temperature. The effects of temperature (4°C and 12°C) and diet composition were studied on the healing process after infliction of mechanical wounds. During the healing process, the changes observed in the epidermis were affected by time, temperature and diet. Elevated temperature and additional dietary Zn accelerated wound healing, as assessed by the mucous cell distribution within the epidermis at two weeks post wounding. The expression of three newly-described salmon chemokines (CK 11A-C) was also found to be modulated in relation to skin damage. The positive effect of dietary Zn shown by histology was positively correlated with an increase in skin chemokine expression. In two experimental trials, Atlantic salmon were fed diets containing functional ingredients and challenged with sea lice. Changes in mucus composition after lice infestation were assessed by proteomics. A large number of proteins (more than 500) were identified in the mucus, many of which were related to immune and stress responses, underlining the importance of skin mucus as a first line of defence in fish. Several of the proteins responded to both sea lice infection and dietary modulation (chemokines, HSPs, AG-2, calmodulin and peptidyl-prolyl cis-trans isomerase). After further validation, these proteins could potentially be used as biomarkers to assess skin functionality. No impact on epidermal thickness or mucous cell area was seen in the challenge trials, indicating that these two structural parameters alone cannot explain the susceptibility to lice attachment. Some of the diets tested did reduce the number of attached lice. Although the reduction was not drastic, it clearly showed the potential of functional diets to modulate lice attachment.